Apr 08, 2013

Study Assesses San Francisco Bay Area Groundwater

Human-made organic chemical constituents less prevalent at high concentrations than statewide

USGS Groundwater Ambient Monitoring and Assessment Program San Francisco Bay

Barium and nitrate were detected at high concentrations in 5% of untreated groundwater used for public water supply in the San Francisco Bay region, while human-made organic chemical constituents were found at high concentrations in less than 1%. These detections are less prevalent than elsewhere in California, according to an ongoing U.S. Geological Survey study of the state's groundwater quality.

This study's findings are significant, because elsewhere in California, high concentrations of inorganic elements generally are found in 10% to 25% of the groundwater used for public supply, nitrate in 1% to 8%, and human-made organic chemical constituents in up to 2%. High concentrations are defined as above the U.S. Environmental Protection Agency's or California Department of Public Health's established maximum contaminant levels (MCLs), or above other non-regulatory health-based levels for chemicals without MCLs. USGS did not analyze treated tap water delivered to consumers. Groundwater is typically treated by water distributors prior to delivering it to customers to ensure compliance with water quality standards for human health.

The San Francisco Bay study was part of the Groundwater Ambient Monitoring and Assessment (GAMA) Program’s Priority Basin Project, a statewide study designed to assess groundwater quality in aquifers that may be used for public water supply and to better understand the natural and human factors affecting groundwater quality. USGS scientists analyzed untreated groundwater from wells in the San Francisco Bay region in 2007, looking for as many as 287 chemical constituents. The San Francisco Bay study region includes the Marina, Lobos, Downtown, Islais Valley, South San Francisco, Visitacion Valley, Westside and Santa Clara Valley groundwater basins in San Francisco, San Mateo, Santa Clara, Alameda and Contra Costa counties.

The volatile organic compounds (VOCs) tetrachloroethene and methyl tert-butyl ether were detected at high concentrations in less than 1% of the San Francisco Bay aquifer system, which is similar to other areas of California. These VOCs are used for industrial purposes, including cleaning during manufacturing and improving gasoline-combustion efficiency.

High concentrations of barium were detected in about 3% of the aquifer system. Barium and other trace elements are naturally present in the minerals in rocks and soils and in the water that comes into contact with those materials. High concentrations of nitrate were detected in about 2% of the aquifer system, and moderate concentrations, found at greater than one-half of the MCL, in about 19%. Elevated concentrations of nitrate generally occur as a result of human activities, such as applying fertilizer to crops or landscaping. Septic systems, as well as livestock in concentrated numbers, also produce nitrogenous waste that can leach into groundwater.

Total dissolved solids, a natural inorganic constituent that affects the aesthetic properties of water, such as taste, color and odor, or may create scaling or staining, were detected at high concentrations in 7% of the primary aquifer system and at moderate concentrations in 37%.

The full report and the accompanying non-technical fact sheet are available online.